1906] SHANTZ—VEGETATION OF THE MESA 207 
The elevated dry ridges are occupied by the Selaginella densa 
formation in which Paronychia Jamesii is a prominent species. 
This evidently represents a younger stage in a succession which will 
give ultimately the Bouteloua formation. Among the most important 
invading species are Stipa comata, Koeloeria cristata, and Bouteloua 
hirsuta, 
If these ridges are sandy, blow-outs often occur which result 
when reclaimed in the Calamovilfa longifolia consocies (fig. 13). This 
consocies thrives much better when not grazed. Grazing changes it 
to the Bouteloua consocies. a 
The Andropogon scoparius evidently represents an earlier stage 
in the formation than the Andropogon furcatus or Bouteloua oligo- 
stachya consocies. It occurs on the hillsides and also is found as an 
early stage in the succession on denuded xerophytic areas. The 
Study of secondary succession would lead also to the belief that 
Muhlenbergia gracillima is only a stage, for here it is replaced 
repeatedly by Bouteloua oligostachya. 
The societies of the formation are, without exception, dominated 
by species which are among the first to invade new areas. Their 
Presence in the formation is largely due to historical reasons. To 
this may be added the fact that the consocies which represent the 
earlier stages, as for example the Bouteloua hirsuta consocies, have 
by far the greater number of societies. 
The Bouteloua oligostachya consocies represents the ultimate 
Stage of the grass formation and is by far the most widely distributed 
of any of the consocies of the formation. 
This study of the Bouteloua formation was suggested by Pro- 
fessor FREDERIC E. CLEMENTs, under whose direction it has been 
carried on. To him and also to Professor Cuas. E, Brssey the 
Writer wishes to express his thanks for many helpful suggestions 
and criticisms. Thanks are also due to the following persons: 
Professor F, D. HEaLp, for advice and criticism; Professor Wm. 
STRIEBY, for generously giving me use of his laboratory for the 
determination of water content; and to Professor F. H. Loun, for 
meteorological data. 
UNIVERSITY oF Missourt, CoLuMBIA. | 
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